Microemulsions 440 INDEX

Y Ishihama, Y Oda, N Asakawa. A hydrophobicity scale based on the migration index from microemulsion electrokinetic chromatography of anionic solutes. Anal. Chem. 68 1028-1032 (1996). 

Even the traditional methods discussed in this chapter can be used for concentrated dispersions through contrast matching. For example, silica particles coated with silane coupling agents in a refractive index-matched mixture of ethanol and toluene can be used in combination with visible probe particles to study the dynamics of particles in dense systems. In the case of microemulsions (Chapter 8), selective deuteration of a component (oil, water, or surfactant) can be used in neutron scattering experiments even to measure the curvature of the oil-water interface. 

FIG. 8.4 Determination of the microenvironment of a molecule (a) a portion of the ultraviolet spectrum of benzene in (1) heptane, (2) water, and (3) 0.4 M sodium dodecyl sulfate and (b) ratio of the intensity of the solvent-induced peak to that of the major peak for benzene versus the index of solvent polarity. The relative dielectric constant is also shown versus the index of polarity. (Redrawn, with permission, from P. Mukerjee, J. R. Cardinal, and N. R. Desai, In Micellization, Solubilization and Microemulsions, Vols. 1 and 2 (K. L. Mittal, Ed.), Plenum, New York, 1976.)... 

Not all emulsions exhibit the classical milky opaqueness with which they are usually associated. A tremendous range of appearances is possible, depending upon the droplet sizes and the difference in refractive indices between the phases. An emulsion can be transparent if either the refractive index of each phase is the same, or alternatively, if the dispersed phase is made up of droplets that are sufficiently small compared with the wavelength of the illuminating light. Thus an O/W microemulsion of even a crude oil in water may be transparent. If the droplets are of the order of 1 pm diameter a dilute O/W emulsion will take on a somewhat milky-blue cast if the droplets are very much larger then the oil phase will become quite distinguishable and apparent. Physically the nature of the simple emulsion types can be determined by methods such as [95] ... 

Since the PS reference sample is almost monodisperse, a cumulant analysis of that material would yield a very small Q, say Q < 0.03. That is, all the correction terms are negligible and Eqs. (17) collapse to Eqs. (12). But cumulant analysis is a useful way to handle practical samples such as pigments, inks, microemulsions, swollen micelles, globular proteins, and spherical virus particles, where there is a size distribution but one that is not very broad (say Q < 0.3). This analysis should be made for the milk data using a non-linem teast-squares fitting of Eq. (17a), neglecting /1.3 and all higher order terms. Report the F, D, and R values as well as the second cumulant /t2 aiid the polydispersity index Q. 

The classification of microemulsions based on size is inadequate. Whether a system is transparent or translucent depends not only on the size but also on the difference in refractive index between the oil and the water phases. A microemulsion with small size (in the region of 10 run) may appear translucent if... 

From this retention factor, the Migration Index (MI) for a compound in microemulsion electrokinetic chromatography was defined as [Ishihama, Oda et al., 1996]... 

The Migration Index scale can be applied to all neutral compounds that migrate in the range 4m and and this might be independent of the volume of the microemulsion. The Migration... 

Fatemi, M.H. (2003) Quantitative structure-property relationship studies of migration index in microemulsion electrokinetic chromatography using artificial neural networks./. Chromat., 1002, 221-229. 

The theoretical description in terms of spherical harmonics also yields a relation between the size polydispersity index p of the microemulsion droplets and the bending elastic constants [43]. The quantity p is accessible by SANS [51, 52, 59-61]. For polydisperse shells as obtained by using deuterated oil and heavy water for the preparation of the microemulsion (contrast variation), one can account for the droplet polydispersity by applying an appropriate form factor, e.g. containing a Gaussian function to model the size distribution [52, 59, 62]. A possible often-used choice is the following form factor... 

The analysis of the light scattering data using CONTIN also allows for a determination of the size polydispersity of the microemulsion droplets, because all the moments u = / pm G(r)rndr which describe the distribution function G(T) are computed (for details see Ref. [98]). The polydispersity index is obtained from... 

The initial structure of die globular or bicontinuous microemulsion is not preserved during polymerization the final system consists in both cases of a dispersion of spherical latex particles with a fairly low index of polydispersity 1.15), as seen from QELS and TEM experiments [48,53,61]. Several factors are responsible for this structural change ... 

The microemulsions containing pilocarpine were formulated using lecithin, propylene glycol, and PEG 200 as co-surfactant and isopropyl myristate as the oil phase. The formulations were of low viscosity with a refractive index lending to ophthalmologic applications. ... 

In some cases, the index of refraction of the spheres can be matched by the index of refraction of the solvent. The scattered intensity should vanish if the spheres are monodisperse, because the contrast would be identically zero at this point. The droplets being actually polydisperse, the residual intensity is a direct measure of the polydispersity. This procedure has been used in light scattering experiments on AOT microemulsions [18]. It can also be used in neutron scattering experiments, where the contrast matching is more easily adjusted by using mixtures of protonated and deuterated oils or water [19,20]. 

Table 1.2. Mean droplet diameter and polydispersity index of o/w emulsions resulting from dilution by water of a microemulsion consisting of a medium-chain triglyceride, soybean phosphatidylcholine, and poly(ethylene gly-col)(660)-12-hydroxystearate, poly(ethylene glycol) 400 and ethanol (from ref. (174) Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley Sons, Inc)...

MLCs also have their place in optical applications, mainly as so-called polymer-dispersed LCs (PDLCs). A PDLC constitutes a microemulsion of an MLC in a film of a conventional (nonPLC) polymer. In the switched off state the MLC and the polymer have different refractive indices, dispersed MLC droplets (not unlike to the islands in PLCs) scatter light quite effectively, and the film is opaque. Then an external electric field is applied, for instance across a capacitor-like metal coating on both sides of the film. The director in all MLC droplets becomes the same. One can choose the MLC + polymer pair so that the refractive index along the director is the same as that of the host polymer. In that case the film in the electric field becomes transparent. Switching the field off and on, one has a light valve with a fairly large area. 

The increasing residual scattering at higher temperatures (see Fig. 9) is straightforward explained by the temperature dependent fluctuations of the interfacial surfactant layer covering the dispersed droplets (26). The fluctuations can be observed since the static contributions of the refractive index increments of the dispersed particles and the solvent (oil) are optically matched. Hence, except for the fluctuations of the surfactant molecules in the monolayer the microemulsions discussed in the present paper show a remarkable monodispersity. 

The sulfonate concentration in the microemulsion was determined from the equilibrated microemulsion phase volume and the known weight of sulfonate in the system the assumption that the microemulsion phase contained all of the sulfonate was justified for all microemulsions. The volume fractions of oil and brine in the microemulsion were determined from the excess volumes of oil and brine, respectively. The microemulsion density and index of refraction needed to calculate the specific refraction (Eq. (1)) were measured on a Mettler-Paar DMA 40 digital density meter with accuracy of 0.0001 g/cm and a Zeiss Abbe refractometer ( 0.0001), respectively the temperature was controlled with an Exacal 100 and Endocal 150 constant temperature circulator-baths connected in series. Interfacial tensions between the microemulsion and equilibrated excess phases were measured on a University of Texas Spinning Drop Tensiometer or a Spinning Drop Tensiometer from S S Instrument Mfg. measurements were carried out until equilibrium values were obtained as indicated by constant readings over a period of at least 1 hour. 

Following the adsorption experiment, the microemulsion was separated from the solids by decantation and successive centrifugations after each centrifugation the microemulsion density was determined and the procedure was repeated until a constant density was obtained. The separated microemulsion was always clear and transparent. The index of refraction was then measured and the specific refraction of the microemulsion was calculated. The interfacial tensions of this microemulsion versus the excess phases... 

All measurements are performed using the refractive index of CdS. In the case of cadmium sulfide nanoparticles produced in the w/o microemulsion the viscosity rj and the refractive index no of the continuous oil phase, namely the xylene-pentanol (1 1) mixture ( = 1.454 cP, D = 1165) are used. Consequently rj and no for water are used when the CdS nanoparticles are redispersed in the aqueous phase. Morphology and size of the redispersed CdS particles are also determined by transmission electron microscopy. Therefore, a small amount of the aqueous solutions is dropped on copper grids, dried and examined in the EM 902 transmission electron microscope (Zeiss) (acceleration voltage 90 kV). The high amount of surfactant brings also difficulties for the preparation of the samples for TEM measurements and consequently samples have to be washed with water to reduce the amount of surfactant. 

Capek et al. polymerized various alkyl acrylates, methyl (MA), ethyl (EA), butyl (BA), hexyl (HA) and 2-ethylhexyl (EHA) acrylate, and alkyl methacrylates in microemulsion [100]. Microemulsion polymerizations of BA and EHA reached in a short time a conversion close to 100%. In case of PMMA the polydispersity index varied from 2 to 4. This can be taken as evidence that the chain transfer events contribute to the termination mechanism [57]. 

The classification of microemulsions based on size is not adequate. Whether a system is transparent or translucent depends not only on the size but also on the difference in refractive index between the oil and the water phases. A microemulsion with small size (in the region of 10 nm) may appear translucent if the difference in refractive index between the oil and the water is large (note that the intensity of light scattered depends on the size and an optical constant that is given by the difference in refractive index between oil and water). Relatively large microemulsion droplets (in the region of 50 nm) may appear transparent if the refractive index difference is very small. The best definition of microemulsions is based on the application of thermodynamics, as discussed below. 

Field-induced reorientation of the director with attendant optical changes has recently been used in a novel application with the potential for large-area LCDs polymer dispersed LCs (PDLCs). A PDLC is a microemulsion of MLC dispersed in a conventional transparent polymer film. In the off state there is a mismatch between the refractive index of the MLC and that of the host polymer film. Hence the dispersion of MLC droplets scatters light very effectively, giving an optically opaque film (Fig. 5.14, left-hand side). On application of an external electric field (across a capacitor-like transparent coating of tin oxide on both sides of the polymer film), the director assumes the same orientation in all of the microdroplets. If the... 

In order to clarify the relation between the phase behavior, interactions between droplets, and the Ginzburg number, we have undertaken further SANS studies of critical phenomenon in a different three-component microemulsion system called WBB, consisting of water, benzene, and BHDC (benzyldimethyl-n-hexadecyl ammonium chloride). This system also has a water-in-oil-type droplet structure at room temperature and decomposes with decreasing temperature. Above the (UCST) phase separation point, critical phenomena have been investigated by Beysens and coworkers [9,10], who obtained the critical indexes, 7 = 1.18 and v = 0.60, and concluded that their data could be interpreted within the 3D-Ising universality. However, Fisher s renormalized critical exponents were not obtained. 

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